Difference Between Radial and Bilateral Symmetry

Main Difference – Radial vs Bilateral Symmetry

Radial symmetry and bilateral symmetry are two different types of biological symmetries found in organisms. The balanced distribution of the body shapes is referred to as the biological symmetry. Biological symmetry can be divided into several categories such as radial symmetry, spherical symmetry, bilateral symmetry, biradial symmetry and the asymmetry. If the body of an organism can be divided into identical halves in any plane, the particular organism is radially symmetric. When an organism’s body is divided by a particular plane and if the resulting left and right sides are the mirror image of each other, that symmetry is called the bilateral symmetry.  The key difference between radial and bilateral symmetry is that radial symmetry generates identical body halves around the central axis whereas bilateral symmetry generates only two sides as left and right.

1. What is Radial Symmetry
      – Definition, Features, Radially Symmetrical Animals and Plants
2. What is Bilateral Symmetry
      – Definition, Features, Bilaterally Symmetrical Animals and Plants
3. What is the difference between Radial and Bilateral Symmetry

What is Radial Symmetry

Radial symmetry gives rise to similar parts, around the central axis of the body. Most animals that produce radial symmetry are symmetrical along the axis of mouth to aboral end. Therefore, the axis which runs from mouth to the adoral end is referred to as the central axis. Roughly identical pieces can be produced by cutting the organism in several planes along the central axis. Hence, any left or right sides cannot be identified within the organism. It only exhibits top and bottom surfaces due to the gravity. Radially symmetrical animals are classified in the taxon: Radiata. Radiata is assembled in different phyla of the kingdom: Animalia. Most animals living in the sea exhibit radial symmetry. Sessile animals like sea anemone, slow moving organisms like starfish and floating animals like jellyfish are examples of radially symmetrical animals. Cnidaria and Echinodermata animals are also considered as radially symmetric.

Other than animals, plants also exhibit the radial symmetry. Identical flower parts such as petals, sepals and stamens occur symmetrically around the flower axis. Female flower parts like carpel, style and stigma often occur in radial symmetry.

Viruses also exhibit radial symmetry by arranging the protein molecules in their coats symmetrically. Types of the viral coats containing the radial symmetry are icosahedrons, polyhedrons, spheres and ovoids.

Bodies of some organisms form segments around the central axis, creating variations in the radial symmetry. Several variations in the radial symmetry can be identified such as tetramerism, pentamerism, hexamerism and octamerism.    

Figure 1: Radial symmetry of the purple sea urchin

What is Bilateral Symmetry

In bilateral symmetry, the body of the organism is divided into two sides as left and right by a basic body plane. Thus, bilateral symmetry is also called the plane symmetry. The plane, which divides the body bilaterally is referred to as the sagittal plane. The two sides generated here are the mirror image of each other. Hence, they exhibit a mirror symmetry in the sagittal plane. The sagittal plane divides the body into left and right vertically. Internal organs may not be symmetrically distributed, but the sense organs and the limb pair can be divided in bilateral symmetry. Since the head is the body part which is located in front of a moving organism, most sensory organs such as eyes and the mouth are concentrated around the head. Thus, moving in one direction generates the front/back difference. The gravity generates the dorsal/ventral difference. But, the left and right are difficult to distinguish.

A majority of organisms, including the human, are bilaterally symmetric. The phylum Echinodermata also contains bilateral symmetry at their larval stage. In plants, some flowers such as orchid and pea families consist of bilateral symmetry.

Figure 2: The bilateral symmetry of Orchid

Difference Between Radial and Bilateral Symmetry

Definition

Radial Symmetry: The organism’s body generates identical sides in any plane which it is divided along the central axis.

Bilateral Symmetry: The body of the organism generates two sides as left and right along the sagittal plane. The two sides are the mirror image of the other.

Divisions into Left or Right Sides

Radial Symmetry: The body cannot be divided into left and right sides.

Bilateral Symmetry: Sagittal plane divides the body into left and the right sides.

Similar Parts

Radial Symmetry: Similar body parts are arranged in a regular manner around the central axis.

Bilateral Symmetry: Similar body parts are arranged in both left and right sides equally. Thus, each side becomes the mirror image of the other.

Development of the Head

Radial Symmetry: The development of a head in front of the body is rare in radially symmetric animals.

Bilateral Symmetry: The development of a head in front of the organism’s body is a prominent feature in bilaterally symmetric animals.

Examples

Radial Symmetry: Sea urchin, sea anemone, Jellyfish, Starfish, Viral coats

Bilateral Symmetry: Human, insects, crustaceans, centipedes, spiders, orchid flowers

Conclusion

Bilateral symmetry is the most spread biological symmetry among organisms. In bilateral symmetry, left and the right side of the body are the mirror image of each other. Hence, external body parts are distributed between the two sides equally. Radial symmetry, on the other hand, generates similar sides around the central axis. Therefore, the key difference between radial and bilateral symmetry is in the different types of body parts they generate along the axis.

Reference:
1. “Symmetry in Biology”. Wikipedia, the free encyclopedia. 2017. Accessed 16 Feb. 2017
2. “radial symmetry.” A Dictionary of Biology. Encyclopedia.com. 16 Feb. 2017
3. “Bilateral (left/right) symmetry”. Undrestanding Evolution. . Accessed 16 Feb. 2017

Image Courtesy:
1. “Withe seaurchin”. By Steven van Tendeloo – (CC-BY-SA-3.0) via Commons Wikimedia
2. “Ophrys apifera”. By Hans Hillewaert – (CC-BY-SA-3.0) via Commons Wikimedia